Acrylic acid is produced from the vapor phase oxidation of propylene or acrolein over an oxidation catalyst. The gaseous reactor effluent is then cooled and/or absorbed in water to obtain an aqueous solution containing from 10-80% acrylic acid, acetic acid, and various impurities.
The separation of acrylic acid from this aqueous stream has been difficult due to the relative volatilities of acrylic acid, acetic acid and water, which prevents simple fractional distillation.
Several different processes have been proposed to perform this separation. For example, U.S. Pat. No. 3,830,707 discloses a process wherein a specific entrainer of isooctane or nitroethane is used to perform an azeotropic distillation. The entrainer allows the removal of acetic acid and water overhead, while substantially purified acrylic acid is removed as a bottoms product. Other entrainers that have been proposed have also included benzene and toluene.
Another method for separation of acrylic acid has been by solvent extraction. The acrylic acid, acetic acid and water are extracted from the aqueous solution by countercurrent extraction. Many different types of solvents have been proposed for this extraction, some of which may also be used as entrainers. Among typical extraction solvents have been phosphates; aliphatic or aromatic hydrocarbons; ethers such as diphenol ether; alcohols and esters. U.S. Pat. No. 3,859,175, although directed to a dual solvent system for entrainment, discloses as prior art the extraction of acrylic acid using a solvent mixture of methylethyl ketone and either a xylene or ethyl benzene. Typically, such solvent extraction systems can be divided into those solvents that have a boiling point higher than acrylic acid and those that boil at a temperature lower than the boiling point of acrylic acid.
A third method for recovery of acrylic acid has been the combination of extraction followed by entrainment such as found in U.S. Pat. No. 3,433,831.
A principal disadvantage with recovery systems using solvents that have a boiling point lower than that of acrlylic acid has been the need for a large amount of heat usage in performing the separation. The present invention has discovered a method for both reducing the size of the final column needed in the separation process and in eliminating the heat necessary for this final separation.